We have undertaken on going program that is aimed at determining, on a molecular level, those genetic alterations in primary breast tumor DNA that have a statistically significant association with the patients history, characteristics of the tumor, and the patients prognosis. The most frequent type of mutation is loss of heterozygosity (LOH) at specific regions of the cellular genome in tumor DNA. In previous studies we have found LOH on chromosomes 1p, Iq, 3p, 7q,11p, 13q, 17p, 17q, and 18q. We have begun to focus on potential target genes affected by LOH. Our current results show that the p53 gene on chromosome 17p13 is altered in 29% of the primary breast tumor DNAs (n=121) examined by the PCR-SSCP technique. The location of the mutations within the gene was evenly distributed in exons 5 through exon 8. We have found that there is a significant association (p=0.003) between tumors having a p53 mutation and those having a high proliferative index as measured by BUDR incorporation. Furthermore, this association appears to primarily refect those tumors having a mutation in either exon 5 (p=0.0002) or exon 6 (p=0.05). There is a body of evidence suggesting that p53 mutations are involved in the process of immortalization of mammalian cells. In other studies we have found that the MCF10A and A1N4 "normal" human breast cell lines, although immortalize for growth in tissue culture, have an unaltered p53 gene. This suggests that either there are other mechanisms by which p53 is inactivated or there are other mutations, independent of those in p53, which cause immortalization of cells in culture. We have previously reported that the NME1 gene on chromosome 17q21 is frequently affected by LOH in primary breast tumors. Others have shown that loss of NME1 expression in breast tumors is associated with a poor prognosis for the patient. We have confirmed this finding. In addition, we have found trend for an association between loss of expression and LOH of the NME1 gene. This association was not perfect, however, suggesting that there could be a closely linked target gene for LOH in this region of chromosome17.